The removal of sulfur oxides and of nitrogen oxides contained in industrial effluents is of increasing importance, particularily in view of acid rain phenomena which are highly detrimental to the environment in industrialized countries and countries proximate thereto.
The main sources of these polluting emissions are the fumes from thermal power plants, industrial furnaces and heaters as well as the effluents from various chemical and refining plants: units for manufacturing sulfuric acid, for calcination of ores, Claus units, catalytic cracking units . . . . Many processes for purifying these effluents have already been proposed and some are industrially used but they all have some disadvantages.
Thus the processes of the wet type, using aqueous solutions or suspensions of carbonates, hydroxides or sulfites of alkali or alkaline-earth metals or of ammonium, have the disadvantage of requiring the cooling of the fumes and their subsequent re-heating before discharge to the atmosphere. Moreover, the formed products--sulfites and sulfates--raise problems of waste disposal for processes with external discharge, or of high cost of treatment for processes with regeneration.
The processes of the dry type, using limestone or dolomite, have also the problem of by-products discharge and generally suffer from insufficient performances.
Some processes of the dry type using regenerable solid absorbents, have also been proposed. Most of them, as the process with alkalinized alumina (U.S. Pat. No. 2,992,884), the process with a mixture of magnesium and aluminum oxides (U.S. Pat. No. 4,529,574) or the process with copper oxide (U.S. Pat. No. 3,778,501), require the use of such reducing gases as hydrogen or a mixture of hydrogen with carbon monoxide to regenerate the absorbent. U.S. Pat. No. 4,423,019 regenerates with hydrocarbon materials.
These techniques have the disadvantage of consuming expensive reducing gases, which strongly affects their economy.
Other processes, as those disclosed in U.S. Pat. Nos. 4,323,544; 3,755,535; 4,283,380; 4,426,365 and Canadian Pat. No. 1,136,384, use, for the solid regeneration, hydrogen sulfide which is an inexpensive reducing agent. This technique has the disadvantage of producing, as gas effluent from the regeneration stage, a relatively corrosive complex mixture containing hydrogen sulfide, sulfur dioxide, sulfur vapor and steam, wherefrom sulfur recovery is relatively difficult. For this recovery it is generally proposed to supply this mixture to a Claus unit, but its variable composition and its corrosivity make the operation very difficult.
U.S. Pat. No. 3,761,570 regenerates by roasting with carbon and oxygen at extremely high temperatures. Other reducing agents may be used, such as elemental sulfur.
British Pat. No. 764,537 discloses an essentially different process comprising the reduction of calcium sulfate or magnesium sulfate by adding coke to expel gaseous sulfur.
U.S. Pat. No. 4,153,535 discloses the use of platinum and palladium to improve the absorption of sulfur oxides by a metallic reactant. The regeneration is performed in the presence of oxygen.
U.S. Pat. No. 4,043,822 discloses the use of a pool of molten sulfur at 250.degree. C. or more, for example 370.degree. C., to convert a metal sulfate residue to sulfites, sulfides, polysulfides and thiosulfates coated with elemental sulfur, which are fully inappropriate to be used as sulfur oxide absorbents.